Objective: Epidemiological studies have reported that an adverse environment during childhood increases the susceptibility to develop ischemic heart disease. We recently reported that early life stress induced a greater Ang II-mediated vascular inflammatory response and elicited an exaggerated Ang II-dependent hypertensive response compared to control rats (Loria et al, Hypertension, 2010). Our research efforts are focused on elucidating molecular mechanism(s) that regulate early life stress-mediated sensitivity to the Ang II pathway. Specifically, we hypothesized that early life stress induces an imbalance in the expression and/or function of vascular AT1 and AT2 receptors leading to enhanced Ang II-dependent vascular inflammation and hypertension in adulthood.
Methods: Using Wistar-Kyoto rats, the maternal separation (MS) protocol was performed separating approximately half of the male pups from their mother 3 hours/day from day 2-14 of life. Normally-reared, littermates were utilized as controls. All rats were subsequently studied as adults (12 weeks old). Aortic reactivity to vasodilators and vasoconstrictors was analyzed. Vascular constriction to Ang II was determined in the presence and absence of endothelium-denudation, L-NAME, AT1, and AT2 specific antagonists, candasartan and PD123319, respectively. Expression of aortic AT1 and AT2 receptors was assessed by real-time PCR analysis. Endothelial nitric oxide synthase (NOS3) activity and expression were determined by the enzymatic conversion of radiolabeled arginine to citrulline and Western blotting in aortic tissue homogenates, respectively.
Results: Vasorelaxation to acetylcholine and sodium nitroprusside were similar in aortic rings from MS and control rats. Aortic constriction to KCl, phenylephrine, and endothelin were similar in both groups of rats. MS enhanced Ang II-induced aortic contraction when compared to control tissue (2 fold, p<0.05). L-NAME significantly increased Ang II-mediated contraction in aorta from both MS and control rats, however the effect of L-NAME was attenuated in aorta from MS rats. Ang II-induced contraction was similar in endothelium-denuded aortic rings from MS and control rats. AT1 receptor antagonism (candasartan, 10µM) abolished the Ang II-induced contraction in both MS and control aortic tissue. AT2 receptor antagonism (PD123319, 1µM) significantly increased the Ang II-induced maximal contraction in control tissue (1.5 fold, p<0.05), however no change was observed in aorta from MS rats. AT1 receptor expression (mRNA) was similar in aortic tissue from both MS and control rats, while AT2 receptor expression (mRNA) was reduced in MS rats. Aortic NOS3 activity and expression were similar in tissues from MS and control rats.
Conclusions: These data indicate that MS induces an Ang II-mediated endothelial dysfunction by decreasing AT2 receptor expression and function leading to enhanced Ang II vasoconstriction. Furthermore, early life stress-mediated endothelial dysfunction may lead to greater sensitivity to increased vascular Ang II levels in adulthood. The study of vascular phenotypic alterations associated with an adverse postnatal environment may contribute to establishing strategies for control of cardiovascular risk in adults.